Usually, the network elements involved in handling a connection or call, do not have information about the state and load conditions of other network elements. For instance, in a GPRS (General Packet Radio Service) or UMTS (Universal Mobile Telecommunications System) network, a SGSN (Serving GPRS Support Node) has no information about the load conditions and state of a GGSN (Gateway GPRS Support Node). The SGSN does neither know if the GGSN is overloaded or not, nor has it information on whether or not the GGSN is out of function because of e.g. a breakdown.
When trying to connect a call or modify an existing connection, the SGSN may for example try to open a PDP (Packet Data Protocol) context to a GGSN that is already overloaded or out of function. In such a case, the GGSN will either return a message rejecting the requested operation, or will not at all return any response.
The GGSN may also reject, when a SGSN is sending e.g. a request for creation or modification of a PDP context e.g. for real-time QoS (Quality of Service), such a request for various other reasons.